Telescope-type levels used in surveying must be leveled accurately before use in taking sightings. This is time consuming but necessary. If then the user slightly bumps the instrument or tripod, or even shifts his weight the leveling must be repeated. Recently automatic compensation for slight misalignment has been built into levels. This compensation is based on pendulating optical elements in the level. This shortens the set-up time and minimizes having to re-level the instrument during use. The telescope is fixed in a level plane and the instrument is used to determine level sights and horizontal angles. The fixed mirror in levels having automatic compensation must be adjusted at the factory. The typical adjustment results in the mirror being suspended, in effect, by adjusting screws. This can be unstable due to differential heat expansion or relaxation of strains in the adjusting means.
A transit is an instrument used for determining horizontal and vertical angles.
A level-transit is a combination instrument which combines both instruments with some loss of precision and magnitude in measuring vertical angles (typically measuring up to 45.degree. elevation or depression). This makes a very useful instrument. When it is used as a level, the telescope is locked parallel to the rotating platform and the instrument must then be just as carefully leveled (the platform is leveled) as with a level not provided with automatic compensation. Automatic compensation has been incorporated in a level-transit but was complicated and was not accepted in the market. It used a fixed and a pendulated prism which resulted in a cumbersome pendulation system.